Kaplan began as a concert pianist, then, beginning in 1941, he wrote music for dozens of Hollywood films. These include Rawhide (1951, with Jeff Corey), Niagara (1953, with Willard Sage), Titanic (1953, with Hamilton Camp) and The Spy Who Came in from the Cold (1965, with Walter Gotell). He was blacklisted for a while in the 1950s after appearing before the House Un-American Activities Committee. During this period he scored the infamous film, Salt of the Earth (1954), produced by fellow blacklisted filmmakers.